Fiber reinforced cement board products used as building materials have been in service since the second decade of the 1900s. Portland cement serves as a matrix or binder for wood particles or strands. In turn, the particles significantly reduce density and contribute strength, particularly tensile strength, to the product. Earlier products were made using wood excelsior as a reinforcing material. Later, asbestos fiber was widely used as a reinforcing fiber. The fiber is intimately mixed into a Portland cement-water slurry so that it is evenly coated. This mixture is predominantly formed into flat panels where the cement is allowed to cure before use. Alternatively, three dimensional products such as corrugated panels, roof tiles, and pipes can be made. Panels can be made with varying densities. Low density products find interior applications as sound absorbent products for walls and ceilings. Higher density panels are used as flooring, siding, sheathing, and concrete forms. For many years asbestos reinforced simulated shingles were widely used as siding for home construction. This application largely disappeared after the health problems associated with asbestos were uncovered. Today, cement board reinforced with cellulose fiber has made a significant comeback as a home siding product. In this application it simulates horizontal or vertical wood siding. Although the product requires special saws, it can otherwise be conventionally handled and nailed. Cement board siding is accepted as an attractive durable, dimensionally stable, low maintenance product resistant to moisture, decay, and insects.
Unbleached kraft pulps are predominantly used as the fiber source for cement board siding. Soroushian et al., in Inorganic-Bonded Wood and Fiber Composite Materials, A. A. Moslemi ed., 3: 9-19 Forest Products Society (1993) (hereafter IBWFCM), generally describe the process of manufacture and properties of the resulting products. Similarly, Soroushian et al., in IBWFCM., 5: 3-7 (1997), describe a process for accelerated curing of the products by autoclaving in high pressure steam. Detailed layouts of plants for fiber reinforced cement board production are given by K. Buchmayer, IBWFCM 6: 99-140 (1998), and G. Agansky, IBFWCM 6: 141-146, (1998). Briefly a slurry of the cellulose fibers is formed. Separately a slurry of cement, silica, filler, and other additives is prepared. These are mixed and formed into sheets or panels, usually on an endless wire screen, where they are then dewatered. The dewatered panels are trimmed, pressed, and stacked. They are then autoclaved to accelerate hydration of the cement and induce at least sufficient strength so that the panels can be handled without breakage. Post curing and finishing are usually additional manufacturing steps before the panels are shipped.
Today, the Hatschek wet process is the most widely used production method. An aqueous slurry of fiber and cement with about 7-10% solids is formed into sheets on several rotating cylinders. Several thin layers are superposed until a panel of the desired thickness is formed. This is dewatered and cured as described above (see Concrete Technology and Design: Natural Fibre Reinforced Cement and Concrete, R. N. Swamy, ed., Vol. 5, pp 23-25, Blackie, London). Typically about 10-30% by weight of the composite material will be refined cellulose fibers with the balance being inorganic mineral components.
The manufacturing environment for cement bonded panels is very highly alkaline. As was noted, unbleached kraft fiber is frequently used as reinforcement. Two problems have been attributed to use of kraft fiber, one during manufacturing and one during use. The first is due to alkaline leaching of materials not removed from the fiber in the pulping process. These materials are generally degraded lignin and carbohydrate residues. When present in excessive amounts they interfere with the curing process and can deleteriously affect strength of the finished product. Under some use conditions the fiber is subject to biological attack also resulting in weakening the product.
The present inventors are aware that some previous consideration has been given to control biological degradation of cellulosic reinforcement in cement board products. They would note that chromated copper arsenate (CCA) treated wood particles have been used. This use has not been with any intention of making biologically durable products but as a way of disposing of scrap or out-of-service CCA treated wood which is not suitable for use as fuel (see Hsu IBFWCM 4: 3-5 (1995), and P. A. Cooper et al. IBFWCM 6: 340-348 (1998)). The authors concluded that CCA treated red pine was useable when comminuted into particles and that the product could be made so that leaching of the toxic materials was minor. Goodell et al., in Forest Products Journal 47(11/12): 75-80 (1997), explored subsoil decay resistance of three wood-cement composite materials. They concluded that only wood particles in the surface regions would likely be subject to fungal attack. Japanese Patent Application 4333611 describes a cross linked acrylic fiber which may be made from monomers that include multivalent metal acrylates. When the multivalent metals in the fiber are copper or zinc the fibers have antibacterial properties. There was no suggested use of the fiber as a cement board reinforcement. Japanese Patent 11-181619 describes a polypropylene fiber useful in cement boards. The fiber is resistant to autoclaving at temperatures as high as 170°-180° C. The fiber is melt spun with a zinc containing nucleating agent, said to contribute antimicrobial properties. Japanese Patent 3132552 describes a cement board fiber containing 3-40% wood fiber having high durability. The fiber is impregnated or coated with a metal compound selected from copper, zinc, aluminum or lead chloride or sulfate. Japanese Patent Application 288149/87 describes wood reinforced cement boards in which a salt of iron, copper, lead, zinc, or aluminum is added to the mixing water. The salt is said to react with components leached from the wood chips and to prevent hardening retardation caused by the leachates. No mention was made of improvement in resistance to biological degradation.
Canadian Patent 1,134,564 describes cellulose fibers which are treated for fungal resistance with metal oxide acylates in which the metal is selected from aluminum, titanium, copper, zinc, antimony, chromium, iron, manganese, or zirconium. Alternatively, other organic and inorganic metal compounds of copper, mercury, chromium, tin, and zinc were said to be useful. The treated fibers are suggested for use as an asbestos substitute in cement products, brake linings, gaskets, etc.
A significant problem with cellulose fibers treated with heavy metal biocides is that they require a high energy input and are subject to considerable degradation during the refining process required for the manufacture of cement board products. The present invention has addressed and presents a solution to this problem.